Rare jellyfish, some as large as the size of a hula hoop, have been spotted off California coasts. Believed to be black jellyfish, the marine mammal has been stinging swimmers over 4th of July weekend, the Associated Press reports.

Black jellyfish, also known as Chrysaora achlyos, have a dark burgundy belly and black tentacles that can grow up to 30 feet long. A kayaker spotted a black jellyfish on July 1 that he described as being the size of a hula hoop. In Laguna Beach, swimmers came out of the ocean with dark membranes stuck to their bodies, AP reports.

"And all of a sudden, I see this big jelly fish swimming right by me," Joe Richman, a kayaker who spotted the rare jellyfish earlier this month in La Jolla, Calif., told CBS 8.

“They are not likely to kill you. They are not a deadly jellyfish, but they can give you a painful sting," Richman said.

Professor Greg Rouse, from the Scripps Institution of Oceanography, agrees. "Oh, if I saw one, I'd be wanting to snorkel all around it. But those tentacles are so long you have to really watch out for them."

Black jellyfish can reach more than 3 feet in diameter and 20 feet in length and have 25-foot-long tentacles. They usually swim in calm, deep waters but every once in a while are spotted in booms off Southern California, according to the Monterey Bay Aquarium.